1. Field of the Invention
This invention relates to an improvement of a method for manufacturing a waveguide circuit element for use in ultrashort wave band.
2. DESCRIPTION OF THE PRIOR ART
In a waveguide circuit element for ultrashort wave bands, it is generally required to minimize the electric loss on the internal surface of a recessed portion of the waveguide circuit element which constitute a conductive surface for transmission of electric waves.
For this purpose, it is of primary importance to process the internal surface of the recessed portion to a great degree of accuracy in the manufacture of the circuit element in order to achieve a minimum electric loss. In processing of the internal surface or electric wave conductive surface of the recessed portion, therefore, it is desirable that the dimensional accuracy of the recessed portion be improved to the order of several microns, that the internal surface be mirror-finished so that the difference in height between the highest and lowest portions of the surface be not more than 0.2 microns except for abnormally high or low portions, and that the internal surface of the recessed portion be made of a metal of as low resistivity as practicable.
In conventional methods of manufacturing a waveguide circuit element with such a recessed portion, the conductive internal surface of the recessed portion is formed either by (1) hollowing out the base metal of copper, phosphor bronze, brass or the like on a machine such as a lathe to form a recess and then effecting an electroplating to coat the internal surface of the recessed portion with a metal of a low resistivity such as silver thereby to form a conductive surface for electric waves, or (2) by effecting a copper electroforming on a metal core of aluminum or the like of a shape similar to the recess and, after extracting it, effecting an electroplating to cost the internal surface of the recessed portion with a metal of a low resistivity thereby to form a transmission surface for electric waves.
Since the above-mentioned conventional methods involve the difficult process of the electroplating for obtaining the conductive surface, the thickness of the deposited metal often varies from point to point on the internal surface of the recessed portion, and accordingly a uniform dimensional accuracy cannot be obtained, which makes it difficult to finish the conductive surface in the dimensional accuracy and mirror-finish described above. Further, the metal deposited on the internal surface of the recessed portion by the electroplating has many defects, resulting in an increased resistivity of the conductive surface for electric waves. In addition, the manufacturing processes are so complicated that the production cost is higher.
As mentioned above, the conventional method of manufacturing waveguide circuit elements with a recessed portion has the disadvantage of low dimensional accuracy and high roughness and resistivity of the internal surface or the conductive surface of the recessed portion, resulting in a high electric loss. Thus, according to the conventional method, it is difficult to manufacture waveguide circuit elements having a conductive surface of a low electric energy-loss.